This application is based on an application No. 2002-096611 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a lighting system for operating a fluorescent lamp with dimming control, and also to a fluorescent lamp.
(2) Description of the Related Art
Compact single-capped fluorescent lamps (hereinafter simply referred to as xe2x80x9cfluorescent lampsxe2x80x9d) are becoming prevalent for their high lamp efficiency as a light source of a lamp apparatus provided at commercial facilities and offices in a buried condition in a ceiling (hereinafter, such a lamp apparatus is referred to as a xe2x80x9cdownlightxe2x80x9d).
There are various types of fluorescent lamps with different lamp powers for various locations and purposes of use. The fluorescent lamps are uniform in the outside diameter of a glass tube constituting a discharge tube and in a nominal lamp current. Each fluorescent lamp, however, differs from others in the lamp power due to a different length of the discharge tube and a different ratio by volume of neon and argon contents sealed within the discharge tube.
For example, when a fluorescent lamp is composed of a discharge tube having an inside diameter of 10.5 mm and a nominal lamp current of 0.32 A, the lamp power maybe made to differ within the range of 16-57 W by varying the length of discharge tube within the range of 68-165 mm and the neon content by volume within the range 30-90%.
Further, when used with an electronic ballast, a fluorescent lamp maybe xe2x80x9cdimmedxe2x80x9d so as to produce a smaller light output over a full range in response to reduction in the lamp current applied thereto. Such a dimmable lighting system is now in wider use.
Generally, electrode filaments of a fluorescent lamp are designed to reach an optimal temperature for thermoelectronic emission upon application of the maximum lamp current (approximately equal to the nominal value). In other words, when the lamp current is reduced for dimming, the temperature of electrode filaments is reduced. To compensate for the temperature reduction, a filament current is supplied to the electrode filaments additionally to the lamp current, so that the temperature of electrode filaments is maintained within an appropriate range.
Unfortunately, when operated at a dimmed level, above fluorescent lamps with high lamp-power (hereinafter, referred to as high-output fluorescent lamps) have following problems although such problems do not occur in fluorescent lamps with low lamp-power. That is, when operated at a dimmed level, ends of the discharge tube are blackened, and the electrode filaments are exhausted, which shortens life of the lamp.
These problems are caused in the following mechanism. In the case of a high-output fluorescent lamp, the neon content is high and thus the cathode voltage drop is large. In synergy with this, the temperature of electrode filaments rises excessively. Because of the excessively high temperature, the thermoelectronic emission material (hereinafter referred to as emitter) coated over the electrode filaments evaporates, and charged particles present around the electrodes are accelerated to cause sputtering of the electrode filaments to a greater extent. As a result, the electrode filaments are more quickly exhausted. In addition, when the lamp current is reduced for dimming, it equally means that the electronic current is reduced as well. As a result, electrons emitted from the surface of the electrode filaments are reduced, so that less cooling effect is achieved.
In view of the above problems, a first object of the present invention is to provide, without upsizing a discharge tube, a lighting system with high lamp-power that free from blackening of the discharge tube ends or loss of the lamp life even when operated at a dimmed level. A second object of the present invention is to provide a fluorescent lamp that is free from blackening of discharge tube ends or loss of the lamp life when used in a lighting system for dimming.
The first object stated above is achieved by a lighting system including a fluorescent lamp and an electronic ballast. The fluorescent lamp is composed of a discharge tube that is formed of at least one glass tube which is bent, and filled with a rare gas containing at least argon. The electronic ballast is for operating the fluorescent lamp with dimming control. An inside diameter of the glass tube is within a range of 12-15 mm. An overall size of the discharge tube is such that a maximum diameter is within a range of 55-70 mm and a maximum length is within a range of 120-220 mm. The electronic ballast applies a nominal lamp current to operate the fluorescent lamp at a full light level. The nominal lamp current is within a range of 0.4-0.5 A. With this construction, the lamp power is increased without involving increase in the ratio of neon content by volume. Therefore, the fluorescent lamp is free from blackening of the discharge tube ends and loss of the lamp life even when operated at a dimmed level. Further, an overall size of the discharge tube is such that a maximum diameter is within a range of 55-70 mm and a maximum length is within a range of 120-220 mm, and thus the discharge tube is applicable to a lighting system employing a downlight. Note that the term xe2x80x9coperation at a full light levelxe2x80x9d means that the lamp is operated by applying the nominal lamp current.
Further, the rare gas may additionally contain up to 75 vol % of neon. With this construction, blackening of the discharge tube ends and loss of the lamp life are suppressed even when the fluorescent lamp is operated at a dimmed level.
Further, each glass tube may be bent to form a substantially U-shape. The discharge tube may be formed of a plurality of the U-shaped glass tubes connected together, and the glass tubes may be arranged to form a polygonal shape in plan view. With this construction, there is no inconsistency in light radiation in the circumferential direction of the discharge tube, so that light is distributed substantially uniformly in the circumferential direction. Here, the term xe2x80x9cplan viewxe2x80x9d refers to the state that the discharge tube is seen from the direction in which the straight portions of the U-shape extend.
Further, the discharge tube may be formed of four U-shaped glass tubes. With this construction, a longer discharge path is formed to improve the lamp output with a size substantially same as a discharge tube formed by, for example, three U-shaped glass tubes.
The second object of the present invention stated above is achieved by a fluorescent lamp that is composed of a discharge tube. The discharge tube is formed of at least one glass tube which is bent, and filled with a rare gas containing at least argon. An inside diameter of the glass tube is within a range of 12-15 mm. An overall size of the discharge tube is such that a maximum diameter is within a range of 55-70 mm and a maximum length is within a range of 120-220 mm. A nominal lamp power at a full light operation is within a range of 0.4-0.5 A.
With this construction, the lamp power is increased with out increasing the ratio of neon content by volume. In addition, the fluorescent lamp is free from loss of the lamp life even when operated at a dimmed level.
Further, the rare gas may additionally contain up to 75 vol % of neon. With this construction, the fluorescent lamp is free from blackening of the discharge tube ends and loss of the lamp life even when operated at a dimmed level.